This invention pertains to the production of a porous diaphragm for use in an electrolytic cell and more in particular to a method to form a porous, organic diaphragm.
Gaseous chlorine has long been produced from sodium chloride in an electrolytic cell having an anode positioned within an anode compartment and a cathode in a cathode compartment spaced apart from the anode compartment by an ion and liquid permeable diaphragm, such as one at least partially formed of asbestos. Other electrolytic cells employ an ion permeable and liquid impermeable ion exchange membrane to separate the anode and cathode chambers. In such electrolytic cells, products including chlorine and sodium hydroxide are formed in the anode and cathode compartments, respectively.
Porous polymeric materials have gained interest over the past few years for use in numerous areas. For example, Canadian Pat. No. 725,704 pertains to a method to calendar mixtures (including polytetrafluoroethylene resins, a liquid capable of wetting the resin and a solid particulate) into porous sheet suitable as filters, separators for batteries and fuel cells, supports for electrode materials, and the like. In the process of British Pat. No. 1,081,046, an aqueous dispersion of polytetrafluoroethylene and a 5 to 30 micron solid particulate additive are formed into a sheet whereafter the particulate additive is leached from the sheet to produce a diaphragm for an electrolytic cell. The sheet of the British Patent can include an inorganic filler, such as barium sulfate, titanium dioxide, amphibol asbestos or serpentine asbestos. Other patents also describe various methods to make and/or use liquid permeable polymeric diaphragms; see for example, U.S. Pat. Nos. 3,627,859; 3,944,477; 4,089,758; 4,098,672 and 4,170,540.
For brevity, polytetrafluoroethylene will hereinafter be referred to as "PTFE".
Electrolytic cells using an alkali metal ion and electrolyte permeable PTFE diaphragm to produce chlorine and an alkali metal hydroxide are substantially the same as prior cells with asbestos diaphragms. For example, the PTFE electrolytic cell includes an anode compartment suited to contain an anolyte such as an aqueous solution or mixture of an alkali metal chloride, for example, sodium chloride, and a cathode compartment adapted to contain a catholyte containing the hydroxide of the alkali metal spaced apart from the anode compartment by the permeable PTFE diaphragm.
Anodes and cathodes are suitably positioned within the respective anode and cathode compartments with a means to supply a direct current to the anode and the cathode suitably electrically connected to these electrodes. The electrolytic cell further includes a means to remove the chlorine produced at the anode from the anode compartment and means to remove the alkali metal hydroxide, and hydrogen if formed, from the cathode compartment.
The PTFE diaphragm separating the anode and cathode compartments is suited to pass a portion of the electrolyte and ions of at least the alkaline metal from the anode compartment to the cathode compartment. This diaphragm is suitably positioned in the electrolytic cell to substantially entirely separate the anode compartment from the cathode compartment.
The diaphragms of the prior art are generally satisfactory; however, there is still a need for a method to produce an improved electrolyte permeable PTFE diaphragm suitable for use in electrolytic cells.